KARACHI, Pakistan — The death toll from a suicide bomb attack on a Sunni Muslim prayer meeting rose on Wednesday to 57 in the southern Pakistani city of Karachi, where officials said they were on high alert for more violence.

Tuesday’s strike by two suspected suicide bombers was the worst ever on Karachi, which has been plagued by sectarian violence and Islamist militant organizations angered by President Pervez Musharraf’s support for the U.S.-led war on terrorism.

“The death toll has now risen to 57, while there are also reports that some people are still missing,” Salahuddin Haider, spokesman of the provincial Sindh government, said.

“Our initial investigations suggest that there were at least two suicide bombers involved in the attack. We have found the body parts, including the heads, of the suspected attackers.”

The attackers struck while worshippers from a Sunni Muslim organisation, Jamaat-e-Ahle Sunnat, had gathered for prayers at the end of a day that marked the anniversary of Prophet Mohammad’s birth.

Among the dead were several locally well-known leaders of Jamaat-e-Ahle Sunnat, which has hundreds of thousands of followers among Pakistan’s dominant Barelvi sect of Sunni Muslims, raising fears of a violent backlash.

Karachi has been one of the main battlegrounds for sectarian violence between Pakistan’s Sunni Muslim majority and minority Shi’ite Muslims over the past two decades.

“It was a very well-planned and co-ordinated attack, and apparently it seems that the attackers were very well-trained,” said Haider, adding that they struck in a way to inflict maximum damage. The Sindh government announced a three-day mourning period, and closed down all educational institutions.

Officials said the government had asked foreign franchises such as the KFC and McDonald’s fast food outlets to remain closed as they are often targeted at such times.

The provincial government has also asked the army stationed in the city to be ready to deploy troops at short notice, according to military officials.

The funeral prayers of two of the victims were held after morning prayers, and more were scheduled for later in the day. No violence was reported during these ceremonies, but the funerals of the Sunni group’s leaders were still to be held.

Tuesday’s explosion was Pakistan’s deadliest since March 19, 2005, when a bomb killed 43 people at a Shiite shrine in the southwestern Baluchistan provincial town of Naseerabad.

On March 2, a suicide bomber who was blocked from driving into the U.S. Consulate instead slammed into an American diplomat’s car, killing the envoy and three others just days before President Bush visited.